svn commit: r252608 - in head: include lib/libc/stdlib

[Andrey A. Chernov]

Author: ache
Date: Wed Jul  3 21:21:54 2013
New Revision: 252608
URL: http://svnweb.freebsd.org/changeset/base/252608

Log:
  1) POSIX requires rand(3) return values to be in the [0, RAND_MAX] range,
  but ACM formula we use have internal state (and return value) in the
  [1, 0x7ffffffe] range, so our RAND_MAX (0x7fffffff) is never reached
  because it is off by one, zero is not reached too.
  
  Correct both RAND_MAX and rand(3) return value, shifting last one
  to the 0 by 1 subtracted, resulting POSIXed [0, 0x7ffffffd(=new RAND_MAX)]
  range.
  
  2) Add a checks for not overflowing on too big seeds. It may happens on
  the machines, where sizeof(unsigned int) > 32 bits.
  
  Reviewed by:    bde [1]
  MFC after:      2 weeks

Modified:
  head/include/stdlib.h
  head/lib/libc/stdlib/rand.c

Modified: head/include/stdlib.h
==============================================================================
--- head/include/stdlib.h       Wed Jul  3 21:14:57 2013        (r252607)
+++ head/include/stdlib.h       Wed Jul  3 21:21:54 2013        (r252608)
@@ -69,7 +69,7 @@ typedef struct {
 #define        EXIT_FAILURE    1
 #define        EXIT_SUCCESS    0
 
-#define        RAND_MAX        0x7fffffff
+#define        RAND_MAX        0x7ffffffd
 
 __BEGIN_DECLS
 #ifdef _XLOCALE_H_

Modified: head/lib/libc/stdlib/rand.c
==============================================================================
--- head/lib/libc/stdlib/rand.c Wed Jul  3 21:14:57 2013        (r252607)
+++ head/lib/libc/stdlib/rand.c Wed Jul  3 21:21:54 2013        (r252608)
@@ -67,15 +67,15 @@ do_rand(unsigned long *ctx)
  */
        long hi, lo, x;
 
-       /* Can't be initialized with 0, so use another value. */
-       if (*ctx == 0)
-               *ctx = 123459876;
+       /* Must be in [1, 0x7ffffffe] range at this point. */
        hi = *ctx / 127773;
        lo = *ctx % 127773;
        x = 16807 * lo - 2836 * hi;
        if (x < 0)
                x += 0x7fffffff;
-       return ((*ctx = x) % ((u_long)RAND_MAX + 1));
+       *ctx = x;
+       /* Transform to [0, 0x7ffffffd] range. */
+       return (x - 1);
 #endif  /* !USE_WEAK_SEEDING */
 }
 
@@ -84,6 +84,10 @@ int
 rand_r(unsigned int *ctx)
 {
        u_long val = (u_long) *ctx;
+#ifndef USE_WEAK_SEEDING
+       /* Transform to [1, 0x7ffffffe] range. */
+       val = (val % 0x7ffffffe) + 1;
+#endif
        int r = do_rand(&val);
 
        *ctx = (unsigned int) val;
@@ -104,6 +108,10 @@ srand(seed)
 u_int seed;
 {
        next = seed;
+#ifndef USE_WEAK_SEEDING
+       /* Transform to [1, 0x7ffffffe] range. */
+       next = (next % 0x7ffffffe) + 1;
+#endif
 }
 
 
@@ -125,6 +133,10 @@ sranddev()
        mib[0] = CTL_KERN;
        mib[1] = KERN_ARND;
        sysctl(mib, 2, (void *)&next, &len, NULL, 0);
+#ifndef USE_WEAK_SEEDING
+       /* Transform to [1, 0x7ffffffe] range. */
+       next = (next % 0x7ffffffe) + 1;
+#endif
 }
 
 
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